1. Field of the Invention
This invention relates to a method of construction and apparatus used in constructing large concrete shells and domes.
2. Description of the Prior Art
It is common practice when constructing roofs, shells, or domes on large concrete structures such as tanks, reactor containment vessels, sewage treatment plants, etc. to erect a complex network of staging filling the area beneath the roof in order to support the roof forms. Concrete with suitable reinforcing is then placed over the forms. This technique requires an extraordinary amount of labor, time, and material, particularly when several identical structures are to be built at the same site.
Systems comprising an attempt at avoiding this waste of manpower and material are set forth in U.S. Pat. No. 3,161,703 which issued on Dec. 15, 1964 to J. J. Watson and U.S. Pat. No. 3,854,690 which issued on Dec. 17, 1974 to O. Heinzle.
For constructing a conical roof on a large concrete tank, Watson proposes a center column for supporting a first set of horizontal radial expandable telescopic beams to underlie a work platform and a second set of downward sloping radial expandable telescopic girders for supporting plywood or other sheet material that serves as the form for a layer of concrete poured thereover. A circular platform on the top of the column forms an upper bearing ring and provides an access opening through which the expandable beams and the center column can be removed after the poured concrete roof has cured.
In the Heinzle system a plurality of radial girders rest on a flanged split collar clamped to a center support column at their inner ends, and each girder has a horizontal flange at its outer end that rests on the wall of the tank. A yieldable foam plastic pad is placed above each flange before the roof is poured. After the concrete roof has cured, the split collar on the support column is loosened and lowered. The inner girder ends are lowered on the collar, with each girder pivoting on the outer flange against the resilient restraint of the yieldable pad. The pad provides a recess to permit withdrawl of the outer flange after the inner end of the girder is free of the flanged collar. The girders can then be withdrawn from the tank through an off-center access opening, but the center column remains a permanent part of the tank.
Other patents of general interest in this art include U.S. Pat. No. 3,072,996 which issued on Jan. 15, 1963 to G. E. McGuire and U.S. Pat. No. 3,427,777 which issued on Feb. 18, 1969 to F. X. Crowley. McGuire discloses the use of radial cables in tension for supporting sector-shaped form panels of plywood or other sheet material over which a layer of concrete is poured. After the concrete has cured, the cables and a temporary support column can be dismantled and removed from the structure. The sector-shaped panels can then be stripped from the roof slab and also removed, if desired. The Crowley patent, on the other hand, discloses a method of fabricating a concrete roof structure by using precast concrete sector-shaped panels that form a permanent part of the final structure.
The foregoing patents, with the exception of Crowley, all use radial girders for supporting individually placed panels of plywood or other sheet material that serve as the skin of the form. U.S. Pat. No. 914,818 which issued on Mar. 9, 1909 to J. H. Finkle, on the other hand, teaches the use of sector-shaped "rafters" as forms for the roof of a silo. The wider end of each rafter is supported by segmental plates attached to an inner collar bearing against the wall of the silo, and the narrow ends of the rafters are secured to a central ring which may be supported by a center column. The upper surfaces of the rafters serve as forms and are covered by a layer of concrete. After the concrete has hardened, the ring may be loosened from its supporting column and disengaged from the narrow ends of the rafters, thus permitting the latter to drop freely to the bottom of the silo.